Hydraulic fracturing is a method of enhancing hydrocarbon production. Hydraulic fracturing is particularly effective in improving the productivity of hydrocarbon reservoirs with low permeability. A non-limiting example of a hydrocarbon formation with low permeability is natural gas from shale beds or other “tight gas” formations. Hydraulic fracturing is also commonly referred to as a “fracture” or “fracturing.”
Hydraulic fracturing treatment operations are typically employed in vertical, deviated and horizontal wells. In a typical hydraulic fracturing operation, the wellbore, also known as a borehole, to be treated or stimulated is drilled through the formation to where the hydraulic fracturing treatment will take place.
Typically, as the well is drilled, it is lined with several layers of steel casing that are anchored to the wellbore with cement. The casing is designed to isolate the borehole, also known as a wellbore, from the surrounding rock. The casing also serves the purpose of preventing fluids in the rocks from entering the wellbore, as well as preventing or mitigating fluids from the formation escaping which can potentially contaminate the area around the wellbore. After the wellbore of interest is drilled, perforating guns are used to “perf” (also referred to as “perforate”) the casing to allow hydraulic fracturing fluid to fracture the formation surrounding the wellbore. A typical example of a cased and perforated wellbore is illustrated in FIGS. 1A 1B and 1C.
At its most basic, hydraulic fracturing is a well stimulation technique to increase productivity of a well by creating highly conductive fractures or channels into the formation surrounding the wellbore. This process normally involves two steps: (1) injecting a fluid at a sufficient rate to pressure the formation to rupture, thereby creating a crack (fracture) in the reservoir rock and (2) placing a propping agent (proppant) to maintain the fracture after the pressure is released or reduced. The goal of hydraulic fracturing is to create a dendritic (branching) pattern of open channels for hydrocarbons in the reservoir to begin flowing into the wellbore. Thus, it would be desirable to determine if such channels have been formed and also the geometry of the channels.
The geometry of a hydraulic fracture affects the success of the fracturing operation. As such, a variety of techniques have been used to attempt to determine fracture geometry. For example, passive seismic methods, also known as hydraulic fracture visualization, have been used to attempt to evaluate the spatial orientation of a fracture and its length. This process has some intrinsic uncertainty because the scatter of natural seismic events can be large compared to the width of the typical fracture. These passive seismic methods are also of limited value because they require a second well for observation, located within a reasonably small distance from the well that is being stimulated (fractured). Unless the observation well is already present, the cost and time delay of drilling the observation well can be prohibitive. Active seismic sources have also been used in an attempt to determine fracture geometry. It has been found that active seismic sources are not efficient because the returned seismic signal will have a small amplitude which can be difficult to detect as the measured fracture is further below the earth's surface. Phrased differently, the signal from an active seismic source may attenuate before the signal can be detected if the fracture is not near the earth's surface. It has also been suggested that use of proppants coated in piezoelectric or magnetostrictive materials could be used to determine fracture geometry. Other authors have suggested that radioactive materials could be injected downhole and tracked as they flow into a fracture. However, if the radioactive materials fail to lodge in the fracture, this technique would be of limited value.